Recent years have seen development of biometric authentication technology for determining whether or not to authenticate an individual, by using biometric information such as a fingerprint or a vein pattern. The biometric authentication technology is widely used for accessibility determination as to a variety of services, such as determination on room entry/exit, judgment on computer accessibility, or user verification in on-line transactions.
In biometric authentication, for example, biometric information of a user is acquired by a sensor, and an image representing the biometric information is used for matching. A biometric authentication device compares a biometric image representing biometric information of a user with a biometric image representing biometric information of a registered user stored in advance, and determines whether the biometric information of the user matches the biometric information of the registered user, for example. When it is determined that the biometric information of the user matches the biometric information of the registered user, the biometric authentication device authenticates the user as the registered user.
Sensors for acquiring biometric information include a contactless sensor capable of capturing biometric information in a manner such that the sensor does not come into contact with a body part containing the biometric information. For example, a contactless vein sensor for capturing a vein pattern of a hand as biometric information is used.
A contactless sensor has a merit that the sensor is hygienic because the user does not have to touch the sensor when biometric information of the user is captured. Further, the contactless sensor is capable of capturing biometric information, without making the user feel restrained. Thus, the contactless sensor has good usability.
On the other hand, a body part containing biometric information can take any posture when capturing the biometric information. In some cases, the posture of a body part containing biometric information with respect to a sensor may be inappropriate for capturing the biometric information. In a worse case, biometric information represented in an image may be distorted. When biometric information represented in an image is distorted, it is difficult or impossible to extract feature points of the biometric information from the image. As compared with a case, in which biometric information is not distorted, a relative positional relationship between feature points may change, and authentication precision may be deteriorated. In view of the above, some techniques for correcting distortion of biometric information represented in an image has been proposed (e.g. see Japanese Laid-open Patent Publication No. 2011-191838 and Japanese Laid-open Patent Publication No. 2007-233981).
For example, Japanese Laid-open Patent Publication No. 2011-191838 discloses detecting a finger shift amount from a correct finger placement position with respect to a finger vein authentication device, from fingertip shape data and finger outline data, for correcting an acquired finger vein pattern.
Further, Japanese Laid-open Patent Publication No. 2007-233981 discloses correcting image enlargement/reduction, parallel displacement, or rotation by carrying out a log-polar transformation of a vein image, or correcting an inclination so that the vein side is aligned perpendicular to the camera optical axis.